1. Field of the Invention
The present invention relates to a double beam optical head. The double beam optical head of the present invention can be applied to an optical head which forms two optical spots on a single information track or a recording medium to perform simultaneous recording and reproduction or to an optical head which forms optical spots on two respectively associated information tracks in a recording medium to perform parallel recording and/or reproduction.
2. Related Background Art
There has been suggested conventional optical heads used in magneto-optical recording/reproducing apparatus, such as an optical head which can perform simultaneous data recording and reproduction (read-while-write) while forming two optical spots on a single information track on a magneto-optical disc or an optical head which can perform parallel data recording and/or reproduction (parallel write/read) while forming optical spots on two information tracks, respectively, in a magneto-optical disc. An example of such optical head is one as described in Japanese Laid-open Patent Application No. 64-82348. FIG. 1 shows the scheme of the optical head as described in the Japanese application. It is an optical head for read-while-write to perform simultaneous recording and reproduction while forming two optical spots on one information track.
In FIG. 1, an optical beam emitted from a semiconductor laser 1 is collimated by a collimating lens 2 and the collimated beam is split into three optical beams by a diffraction grating 3. These optical beams pass through a beam shaping prism 5 and a beam splitter 6 and are then condensed by an objective lens 7 so as to be focused on a magneto-optical disc 8. Light reflected on the magneto-optical disc 8 is again condensed by the objective lens 7 and is then reflected by the beam splitter 6. The reflected light passes through a half wave plate 10, a condenser lens 11 and a beam splitter 12 to be received by photodetectors 13, 14. A servo signal and an RF signal are obtained from outputs from the photodetectors 13, 14. Numeral 9 designates a magnetic head which can apply a modulation magnetic field to the magneto-optical disc 8.
Light spots formed on the magneto-optical disc 8 are located on a same information track in the graph as shown in FIG. 2. Namely, an optical spot SP1, corresponding to an optical beam on the optical axis of the objective lens 7, has a larger quantity of light than the two other spots SP2, SP3. In this conventional example the optical spot SP1 is zeroth order diffracted light emerging from the diffraction grating 3. Thus, the optical spot SP2 corresponding to first order diffracted light has a smaller quantity of light than the optical spot SP1. The two optical spots SP1, SP2 are so arranged that the optical spot SP1 advances and the optical spot SP2 follows on the same information track moving in the direction as shown by the arrow. In recording information the optical spot SP1 is formed with a predetermined recording power and the magnetic head 9 applies a magnetic field modulated by data information to a recording area. At the same time, reproduction is conducted with the optical spot SP2 to check (or verify) recorded information just after the recording with the optical spot SP1. Also, the optical spot SP1 is formed with a reproduction power in normal information reproduction.
In the optical head as described above, a reproduction signal for verification by the optical spot SP2 is used to check a recorded signal. It is, therefore, preferred that the quality of reproduced signal for verification is kept at a same level as that of the reproduced signal in normal reproduction. In the conventional example the three optical beams are separated from each other at the diffraction grating 3 and start traveling at different angles. Generally, the distribution of a beam incident on the objective lens can be assumed to have a substantially Gaussian distribution. Then the optical beams entering the objective lens 7 have respective intensity distributions as shown in FIG. 3. The intensity center of distribution Bm1 corresponding to the optical spot SP1 is coincident with the optical axis of the objective lens 7, while the intensity center of distribution Bm2 corresponding to the optical spot SP2 deviates by a distance d from the optical axis of the objective lens 7. In other words, a deviation amount .delta., which is a distance from the optical axis of the objective lens 7 to the center of intensity distribution of optical beam, is 0 for distribution Bm1 and d for distribution Bm2.
FIG. 4 shows a relation between a deviation amount .delta. and the size of the optical spot in the direction of deviation. In the conventional example as described above, .delta.=d for distribution Bm2 corresponding to the optical spot SP2, which corresponds to C as a spot diameter of optical spot SP2, relatively large as compared with B for the spot diameter of optical spot SP1. Accordingly, the quality of the reproduced signal from the optical spot SP2 is inferior to that from the optical spot SP1.
This is also the case in read-while-write or in parallel record/reproduction without using the diffraction grating but using array lasers to form two optical beams.